In a magnetic recording device in which a read head is based on a spin valve magnetoresistance (SVMR) or a giant magnetoresistance (GMR) effect, there is a constant drive to increase recording density. One method of accomplishing this objective is to decrease the size of the sensor element in the read head that is suspended over a magnetic disk on an air bearing surface (ABS). The sensor is a critical component in which different magnetic states are detected by passing a sense current through the sensor and monitoring a resistance change. A popular GMR configuration includes two ferromagnetic layers which are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer wherein the magnetization vector can rotate in response to external magnetic fields. In the absence of an external magnetic field, the magnetization direction of the free layer is aligned perpendicular to that of the pinned layer by the influence of hard bias layers on opposite sides of the sensor stack. When an external magnetic field is applied by passing the sensor over a recording medium on the ABS, the free layer magnetic moment may rotate to a direction which is parallel to that of the pinned layer. Alternatively, in a tunneling magnetoresistive (TMR) sensor, the two ferromagnetic layers are separated by a thin non-magnetic dielectric layer.
A sense current is used to detect a resistance value which is lower when the magnetic moments of the free layer and pinned layer are in a parallel state. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the sensor stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head in which the cross-sectional area of the sensor is typically smaller than 0.1×0.1 microns at the ABS plane. Current recording head applications are typically based on an abutting junction configuration in which a hard bias layer is formed adjacent to each side of a free layer in a GMR spin valve structure. As the recording density further increases and track width decreases, the junction edge stability becomes more important so that edge demagnification in the free layer needs to be reduced. In other words, horizontal (longitudinal) biasing is necessary so that a single domain magnetization state in the free layer will be stable against all reasonable perturbations while the sensor maintains relatively high signal sensitivity.
In longitudinal biasing read head design, films of high coercivity material are abutted against the edges of the sensor and particularly against the sides of the free layer. By arranging for the flux flow in the free layer to be equal to the flux flow in the adjoining hard bias layer, the demagnetizing field at the junction edges of the aforementioned layers vanishes because of the absence of magnetic poles at the junction. As the critical dimensions for sensor elements become smaller with higher recording density requirements but sensor layer thickness decreases at a slower rate, the minimum longitudinal bias field necessary for free layer domain stabilization increases.
A high coercivity in the in-plane direction is needed in the hard bias layer to provide a stable longitudinal bias that maintains a single domain state in the free layer and thereby avoids undesirable Barkhausen noise. This condition is realized when there is a sufficient in-plane remnant magnetization (Mr) from the hard bias layer which may also be expressed as Mrt since hard bias field is also dependent on the thickness (t) of the hard bias layer. Mrt is the component that provides the longitudinal bias flux to the free layer and must be high enough to assure a single magnetic domain in the free layer but not so high as to prevent the magnetic field in the free layer from rotating under the influence of a reasonably sized external magnetic field. Moreover, a high squareness (S) hard bias material is desired. In other words, S=Mr/MS should approach 1 where MS represents the magnetic saturation value of the hard bias material.
Referring to FIG. 1, a generic TMR or CPP-GMR read head structure 40 is shown that is similar to read heads currently being employed in manufacturing or in development. Read heads of this type are described in the following references: R. Fontana and S. Parkin, “Magnetic tunnel junction device with longitudinal biasing”, U.S. Pat. No. 5,729,410; S. Mao et al., “Commercial TMR heads for hard disk drives: characterization and extendibility at 300 Gbit/in2”, IEEE Trans. Magn., Vol. 42, No. 2, p. 97 (2006); and T. Kagami et al., “A Performance Study of Next Generation's TMR Heads Beyond 200 Gb/in2”, IEEE Trans. Magn., Vol. 42, No. 2, p 93 (2006). The read head 40 is comprised of a bottom shield 1 and a top shield 16 that also function as bottom and top electrical leads for conducting current through the sensor stack. Layers 6 through 10 represent the sensor stack that is formed on the bottom shield 1 and is patterned by a well known method to form nearly vertical or slightly sloping sidewalls such that layer 6 has a larger length along the x-axis than layer 10. Layer 6 is generally a multilayer structure that may have a seed layer, an anti-ferromagnetic (AFM) layer, and a pinned layer (not shown), being exchange coupled to the AFM layer, deposited sequentially on the bottom shield 1. A reference or second pinned layer 7 is on the layer 6 and may have a synthetic anti-ferromagnetic (SAF) composition like the first pinned layer. The spacer layer 8 is usually an insulator comprised of a metallic oxide for TMR heads, or a metallic layer or a layer with metallic nano-channels for CPP GMR heads. Above the spacer layer 8 is a free layer 9 and a layer 10 that may be a capping layer, for example.
An insulating layer 3 is formed along the sidewalls of the sensor stack and on the bottom shield 1 adjacent to the sensor stack and is typically a metallic oxide that prevents shorting between the top and bottom shields as well as the sensor stack. A seed layer 4 disposed on the insulating layer 3 is commonly used to promote the in-plane easy axis of the hard bias material during deposition of the hard bias layer 5 as mentioned in the following references: D. Larson et al. in U.S. Pat. No. 7,061,731; P. Chau et al. in Publication No. US 2005/0066514; H. Gill in Publication No. US 2006/0114622; M. Pinarbasi in Publication No. US 2006/0087772; and K. Zhang et al. in Publication No. US 2006/0132989. The seed layer 4 leads to the hard axis of the hard bias (HB) layer 5 being grown perpendicular to the seed layer plane. Above the HB layer 5 there is generally a capping layer 11 that has multiple purposes such as increasing the spacing between the hard bias layer 5 and top shield 16, increasing the HB field uniformity, and reducing HB internal stress after deposition.
In an ideal situation, the magnetization of the hard bias layer 5 is aligned longitudinally (along the x-axis) and parallel to the sensor film plane after the sensor is exposed to a strong magnetic field applied in the direction of the arrows 12. This process is called the hard bias initialization step. Ideally, the magnetic charges 13a, 13b, created by the HB magnetization on the side edges of the sensor stack will provide a longitudinal field in the free layer as a bias field. This field keeps free layer 9 magnetization longitudinal when no external field is applied. In the example shown in FIG. 1, the reference layer 7 has a fixed magnetization along the y-axis as a result of an annealing process and coupling with the first pinned layer and AFM layer in layer 6. When a magnetic field of sufficient strength is applied in the y direction from a recording medium by moving the read head 40 over a hard disk surface (not shown) oriented in the z direction, then the magnetization in the free layer 9 switches to the y directions. This change in magnetic state is sensed by a voltage change due to a drop or increase in the electrical resistance for an electrical current that is passed through the sensor. In a TMR or CPP spin valve, the sense current between the top shield 16 and bottom shield 1 is in a direction perpendicular to the planes of the sensor stack.
Unfortunately, the ideal case of forming magnetic charges adjacent to the sensor stack as depicted in FIG. 1 is not representative of conditions in an actual read head since the read head 40 has some intrinsic properties that tend to cause significant HB field degradation and large HB field variations. These problems are especially significant in narrow shield-shield read head applications where the distance between the top and bottom shields is reduced in order to achieve high areal density magnetic recording.
Referring to FIG. 2a, the growth pattern of the HB material in HB layer 5 is shown for the longitudinal HB scheme described in FIG. 1. Only the left portion of the sensor structure is shown in the drawing. Note that the insulating layer 3 has a section 3a that is formed on the bottom shield 1 and a section 3b along the side of the sensor stack. Since the seed layer 4 conforms to insulating layer 3 and promotes an in-plane easy axis orientation, the easy axis of the HB layer 5 formed on the seed layer above section 3a is parallel to the sensor film plane while the easy axis of the HB layer deposited on the seed layer on sloped section 3b will generally follow the slope direction. Arrows 14a and 14b illustrate the possible growth pattern of the HB easy axis above sections 3a and 3b, respectively. Area 15 where the two different easy axis orientations meet is where a high stress or amorphous phase of the HB layer 5 will likely occur.
After HB initialization, HB magnetization will fall back on to the easy axis directions shown in FIG. 2a since the HB material usually has a strong uniaxial anisotropy along the easy axis. Therefore, the region of the HB layer 5 adjacent to section 3b will contribute little to the bias field in the free layer 9 as its magnetization direction is mainly along the sensor edges and does not produce much magnetic charge on the stack edges, unlike the ideal case in FIG. 1. On the other hand, area 15 where two differently oriented magnetizations meet in the HB layer 5, will have body charges that can produce a magnetic field to the sensor free layer 9 as a biasing field. Thus, in reality, the hard bias field present in conventional TMR or CPP GMR read heads is theoretically not comprised primarily of surface charges along the sensor edge, but is from body charges in the HB layer 5. The farther away the body charges (not shown) are from the sensor edge, the less amount of biasing field that the charges can produce in the free layer 9, because the solid angle 50 from the charged areas relative to the free layer edges becomes smaller as distance between the body charges and free layer increases.
In FIG. 2b, the concept of solid angle is explained in more detail and the layers in read head 40 are removed to simplify the drawing. This cross-sectional view illustrates that a charged surface 51 in the HB layer can be projected 54 onto a spherical surface 52 having an area “s” and ultimately focused to a point 53 on the free layer that is located a distance r from the spherical surface. From a top-down view (not shown), the spherical surface would appear as a circle, and from a cross-sectional view, the projection of the charged surface 51 onto point 53 would appear as a cone shape. The solid angle 50 is determined by dividing s by r2.
A reader shield-HB coupling effect can further degrade the HB strength according to the scenario depicted in FIG. 2a. In FIG. 3a, an example is shown where there are no magnetic shields above or below the sensor stack. Two circled regions 17a, 17b are schematics of body charge distribution in the HB layer 5. Exchange interaction between the HB grains is also included because HB grains in read heads are usually not well segregated by non-magnetic boundaries as they are in a magnetic recording medium. Therefore, the tilting of the magnetization away from the x-axis or sensor plane occurs farther away from the sensor edge and insulating layer section 3b. The magnetization direction in the HB layer 5 is represented by arrows 18. Region 17a represents a stronger body charge and region 17b has a weaker body charge. When top and bottom shields are added, especially when the top shield 16 (FIG. 3b) follows the HB layer 5 and sensor stack topography, the HB layer strength can be weakened by the shield-HB coupling of the HB and HB magnetization rotation.
FIG. 3b illustrates the effect of shield-HB coupling. The image of the HB in the top shield, for example, dashed arrows 19, has two effects. First the induced surface charge on the lower surface 16a of the top shield 16 facing the HB layer 5 is opposite to the HB surface charge or body charge that provides the hard bias field to the free layer 9 and thereby causes the effective bias field to the sensor stack to decrease. Secondly, the HB image (arrows 19) attracts HB magnetization 18 to rotate towards the direction perpendicular to the bottom surface 16a of the top shield to minimize Zeeman energy between the HB image and HB layer 5. Additionally, with the top shield 16 conforming to the HB layer 5 topography, the imaging effect is equally strong along the top surface of the HB layer which produces more canceling charges (not shown) on the bottom surface 16a and also enhances the rotational behavior of the magnetization direction 18. Additional rotation of the HB magnetization from shield-HB coupling will cause the body charges in the HB layer 5 to migrate further inside the HB layer and away from the sensor stack edge. Note that region 17a has weaker charges in FIG. 3b than in FIG. 3a and there is an additional region 17c having weak charges formed a greater distance from section 3b and the sensor structure. As a result, the effective solid angle of the HB body charge relative to the free layer 9 becomes smaller and the HB field decreases correspondingly. Note that region 17b has a smaller solid angle 50b than the solid angle 50a for region 17a because of a larger distance “r” from the free layer 9. Effective solid angle is related to charge density (body integration)×solid angle of the area divided by the MS of the hard bias material.
The shield-HB coupling mechanism is more severe in narrow shield-shield spacing examples. For high density magnetic recording beyond 1Tb/in2, the areal density requires increasingly narrow down-track bit length and cross-track track width. To successfully read back narrower bit lengths, the read head's down track resolution must be improved which is usually achieved by narrowing the reader shield-shield spacing. In FIG. 4, an example of narrow shield-shield spacing is shown. To narrow the shield-shield spacing between bottom shield 1 and top shield 16, a very thin capping layer 11 and a thin HB layer 5 are generally employed. When a thin capping layer 11 is used, the distance between the HB layer 5 and bottom surface 16a is reduced thereby leading to enhanced imaging of the HB layer by the top shield 16 and a greater shield-HB coupling effect. In a scheme with a thinner HB layer 5, the HB cross section area decreases proportionally with the HB thickness. As a result, the body charge that contributes to the bias field is reduced. Therefore, a narrower shield-shield spacing in a conventional read head causes degradation of the HB field and a loss in device performance because of the combined effect of thinner capping and HB layers.
The shield-HB coupling induced HB field weakening can be mitigated by flat top shield 16 topography. However, flat topography normally requires chemical mechanical polishing (CMP) of the sensor stack. As sensor shield-shield spacing shrinks to several tens of nanometers in advanced technologies, it is both technically and economically difficult to control CMP of the sensor stack with high fabrication yield. For instance, non-uniformities in the CMP process can easily cause large thickness variations in the capping layer 10 from one sensor to the next. An alternative HB scheme is needed that avoids CMP and can provide a robust and stable biasing field to the free layer in the sensor stack even in narrow shield-shield spacing configurations.
During a search of the prior art, the following references were discovered. U.S. Patent Application 2006/0132989 teaches longitudinal in-plane biasing. U.S. Patent Application 2005/0237677 describes a Co based hard bias layer formed on an underlayer made of Ru, Ti, Zr, Hf, Zn, or an alloy thereof. In this CIP design, however, uncompensated back side charges will degrade the HB field generated by charges inside the HB layer.
U.S. Pat. No. 7,061,731 discloses an oblique deposition of a hard bias layer in a direction normal to the preferred direction of anisotropy. A seed layer is optional. U.S. Pat. No. 6,858,320 teaches that a seed layer may degrade the orientation of the underlayer so a non-magnetic intermediate layer is preferred. U.S. Patent Application 2006/0114622 describes a hard bias layer formed on a seed layer on either side of a sensor. An AP pinned structure on the hard bias layer reduces dependence on the seed layer and increases coercivity. U.S. Patent Application 2006/0087772 shows a hard bias layer formed on a CrMo seed layer. U.S. Pat. No. 7,072,156 discloses a decoupling layer between two hard bias layers that acts as a seed layer to cause grains to have easy magnetization parallel to the interface between the layers. U.S. Patent Application 2005/0066514 shows a hard bias seed layer made of Si and Cr or CrMb.
U.S. Pat. No. 6,185,081 describes a seed layer that promotes in-plane c-axis growth. U.S. Pat. No. 6,144,534 discloses a seed layer that disconnects the coherent crystal growth of the c-axis toward the perpendicular. U.S. Patent Publication 2005/0164039 teaches that the c-axis should be in-plane and not perpendicular.